Major airports and way points presently are equipped with secondary surveillance radars (SSRs) such as used in the National Air Traffic Control Radar Beacon System. These SSRs are designed to cooperate with transponders carried on aircraft to provide for the transmission of identification and other data, such as altitude data, from the aircraft to the SSR. An air traffic controller observing a radar display directs the pilots of an involved aircraft by radio, usually with voice communication, so as to maintain or restore safe separations between aircraft. Such a system is limited in capability because each aircraft must be dealt with individually and requires its share of the air traffic controller's time and attention. When traffic is heavy, or visibility is low, a possibility of collision increases.
Current SSR systems transmit a beacon at 1030 MHz. The beacon includes three pulses of 0.8 microsecond durations with a first and last pulse, P1 and P3, being 8 or 21 microseconds apart. The SSR beam normally has an azimuthal beamwidth of from 3 to 4 degrees. If the P3 pulse is delayed 8 microseconds from the P1 pulse, then the SSR is requesting the identity of a receiving aircraft. This pulse delay scenario is called Mode A. If the P3 pulse is delayed 21 microseconds from the P1 pulse, then the SSR is requesting the altitude of the receiving aircraft. This pulse delay scenario is called Mode C.
The P2 pulse is transmitted two microseconds after the P1 pulse and between the P1 and P3 pulses. The P2 pulse is transmitted from a co-located omnidirectional antenna and is compared in amplitude with the P1 pulse in a transponder located on an aircraft to suppress responses to undesired sidelobes created when the P1 and P3 pulses are transmitted from a directional antenna. Only when the amplitude of the received P1 pulse exceeds that of the received P2 pulse does the transponder on the aircraft radiate a response.
A secondary surveillance radar in a particular geographic location has a locally unique pulse repetition frequency (PRF) which can vary between 250 and 450 pulses per second in intervals of 4 or 5 Hz. The PRF for the secondary surveillance radar is locally unique to aid in air traffic "defruiting" (discriminating synchronous garble produced by other SSRs).
In general, each aircraft transponder is sequentially interrogated in two modes: one for its identity, Mode A, and the other for its altitude, Mode C. The replies to these alternating interrogations are demodulated at the SSR ground site and data are linked to an Air Traffic Control Center for evaluation. Similar to the case for secondary surveillance radar PRF, the alternating sequence of Mode A, Mode A and Mode C, or Mode S transmissions are unique for each SSR in a particular geographic location.
The transponder signal includes two framing pulses which are spaced 20.3 microseconds apart. The interval between the framing pulses includes a number of discrete sub-intervals, in each of which a pulse may or may not be transmitted.
The sequence of sub-intervals containing data define the type and content of the information being transmitted. Twelve binary code groups, each representing one or more pieces of information such as identification, barometric altitude and/or a distress signal. The desired four digit identification code group may be set into the transponder by the pilot of the aircraft using an analog wheel switch, or in some cases automatically or semi-automatically.
The first framing pulse of the reply of a transponder follows the end of a received interrogation by a standard delay of 3 .mu.s plus or minus 0.5 .mu.s . Each of the data-carrying pulses has a width of 0.45 .mu.s. The minimum interval between pulses is 1.0 .mu.s. The second framing pulse is transmitted 20.3 .mu.s after the first framing pulse. The transponder is then disabled for a period of about 125 .mu.s called the "dead time."
Of the over 271,000 aircraft licensed to operate by the FAA, approximately 80 percent are equipped with transponders which reply to interrogations received from SSR ground stations. Within the continental U.S., most above-ground-level altitudes higher than 2000 feet are "illuminated" by one or more SSRs. In some areas, as many as thirty SSRs may be within a line-of-sight. As each SSR beam sweeps past an aircraft, it interrogates that aircraft's transponder from approximately 15 to 25 times at intervals of about 2 to 5 milliseconds. Each secondary surveillance radar transmits a pulse at 1030 MHz, which elicits a reply transmission from the transponder at a frequency of 1090 MHz.
Current transponders are designed to transmit an identification code (Mode A) and in some cases the altitude (Mode C) of the instrumented aircraft. It is estimated that only 40 percent of all transponder-equipped aircraft have altitude-transmitting transponders. However, the FAA recently has increased the number of Terminal Control Areas requiring Mode C transponder equipped aircraft from 13 to over 200. In addition, the state of California recently has passed legislation mandating the use of Mode C transponders statewide. Thus, many more aircraft will be required to have Mode C capable transponders in the near future.
A Mode S interroqation beacon system (SSR) has also been developed for use in the future. The Mode S beacon system was developed as an evolutionary improvement to the ATCRBS system to enhance air traffic control surveillance reliability and to provide a ground-air-ground digital data communication capability. Each aircraft will be assigned a unique address code which permits data link messages to be transferred along with surveillance interrogations and replies.
Like ATCRBS, the Mode S system will locate an aircraft in range and azimuth, report its altitude and identity, and provide the general surveillance service already available. However, because of its ability to selectively interrogate only those aircraft within its area of responsibility, Mode S can avoid the interference which results when replies are generated by all of the transponders within the beam. If Mode S schedules its interrogations appropriately, responses from aircraft will not overlap each other at the receiver.
Several variants of airborne proximity indicator and collision avoidance systems have been proposed in the last three decades. The Traffic-Alert/Collision Avoidance System (TCAS) as proposed by the Federal Aviation Administration is a set of system specifications which attempts collision avoidance by having an aircraft-mounted beacon (similar to an SSR) interrogate and stimulate a coded response from surrounding transponder-equipped target aircraft. Target location is determined via a determination of polar timing and direction finding.
A collision-avoidance/proximity-warning system is disclosed in U.S. Pat. No. 4,027,307 issued to Litchford, for determining passively the range and bearing of those mobile vehicles within a selectable proximity to an observer's mobile vehicle position from interrogation replies of the target mobile vehicle transponders and the interrogation of a secondary surveillance radar. This system employs a direction-finding antenna and circuits for indicating the angle of incidence of transponder replies and means for determining the slant range to the replying transponders dependent on the time of receipt of the transponder replies relative to SSR interrogation reception. The slant range may be determined either entirely passively from a bearing measurement and measurement of the difference between the time of receipt of the interrogation and a corresponding transponder reply as long as at least two SSR beams are being interrogated, or is determined actively by transmitting interrogation signals and measuring the time difference between the transmission and target-aircraft transponder reply in a manner similar to TCAS.
The '307 Litchford patent utilizes an amplitude sensitive direction finder to determine the bearing of sensed signals and determines the location of the owned aircraft and the intruder aircraft based solely on the present values of sensed parameters.